[coop] make mono_gc_init_finalizer_thread GC Unsafe aware.

[lewurm]

Usages in our SDKs:

• .NET 5 sample: https://github.com/dotnet/runtime/blob/c8b9cf1bf6654d5d1ad3fdab1a966590d707de4e/src/mono/netcore/sample/iOS/runtime.m#L282-L284
• mono SDK sample:

  mono/sdks/ios/app/runtime.m

  Lines 340 to 342 in 2709c81

  	MONO_ENTER_GC_UNSAFE;
  	mono_gc_init_finalizer_thread ();
  	MONO_EXIT_GC_UNSAFE;

• Xamarin.iOS: https://github.com/xamarin/xamarin-macios/blob/04acaf65e4ba90ac6e2b37fcc19a50fbc818a9c4/runtime/monotouch-main.m#L206-L208

It would be nice to move those macros into mono_gc_init_finalizer_thread (). However, we aren't sure how much it will break. I'll leave it up to @lambdageek.

/cc @alexischr @akoeplinger

[alexischr]

To be clear, mono_gc_init_finalizer_thread cannot avoid entering GC-Safe mode - it creates a thread object on the managed heap.

@thaystg Could you discuss the MONO_ENTER_GC_SAFE_UNBALANCED; line in #16907 ? Is it a necessary fix for a certain embedded configuration? If that line is not necessary, we can remove all of this.

[thaystg]

OKay, the reason why I changed this is because sometimes with embedded mono we can call mono_jit_init_version or mono_jit_init and doesn't start the execution of a managed code yet, and then we try to suspend the vm to run the GC, in this case we got a hang.
To fix this what I tried to do is, enable MONO_ENTER_GC_SAFE_UNBALANCED when it runs mono_jit_init, so we don't need to care about this thread because it's not executing a managed code, until we run mono_runtime_invoke or mono_runtime_invoke that will run a MONO_ENTER_GC_UNSAFE_BALANCED and then it will be needed to stop when we run the GC.

[thaystg]

I think that we can move the MONO_ENTER_GC_UNSAFE and MONO_EXIT_GC_UNSAFE inside the mono_gc_init_finalizer_thread, mono_gc_init_finalizer_thread is an external only function and will be the same behavior that is already implemented by mono_runtime_invoke because both of them will call managed code.

[lambdageek]

Fixed by dotnet/runtime#48465